When an outside air temperature is low, leading to a reduction in the temperature of a working oil (to be referred to as “ATF” hereafter) of an automatic transmission, a viscosity of the working oil increases, causing delays in the engagement/disengagement of frictional engagement elements that contribute to shifts, such as clutches and brakes. As a result, shift shock and shift delays occur.
To prevent this, JP2-96068U discloses a technique of prohibiting shifts to a highest gear position at a low temperature and waiting for a predetermined time to elapse following engine startup before canceling the prohibition of shifts to the highest gear position. According to this technique, by prohibiting shifts to the highest gear position, a rotation speed of the engine is maintained at a higher speed than when shifts to the highest gear position are not prohibited, and therefore increases occur in an amount of ATF stir in a torque converter and a rotation speed of a pump for circulating the ATF. As a result, an increase in the temperature of the ATF can be promoted.
JP7-174223A discloses, with a similar aim, a technique of prohibiting engagement of a lockup clutch (also known as a damper clutch) provided in a torque converter at a low temperature until a cumulative value of an engine rotation speed exceeds a predetermined value. According to this technique, the ATF continues to be stirred in the torque converter, and therefore an increase in the temperature of the ATF can be promoted.